KVM: MMU: Don't flush shadow when enabling dirty tracking

Instead, drop large mappings, which were the reason we dropped shadow.
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/mmu.c

@@ -3538,14 +3538,18 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 		if (!test_bit(slot, sp->slot_bitmap))
 			continue;
 
-		if (sp->role.level != PT_PAGE_TABLE_LEVEL)
-			continue;
-
 		pt = sp->spt;
-		for (i = 0; i < PT64_ENT_PER_PAGE; ++i)
+		for (i = 0; i < PT64_ENT_PER_PAGE; ++i) {
+			if (sp->role.level != PT_PAGE_TABLE_LEVEL
+			    && is_large_pte(pt[i])) {
+				drop_spte(kvm, &pt[i],
+					  shadow_trap_nonpresent_pte);
+				--kvm->stat.lpages;
+			}
 			/* avoid RMW */
 			if (is_writable_pte(pt[i]))
 				update_spte(&pt[i], pt[i] & ~PT_WRITABLE_MASK);
+		}
 	}
 	kvm_flush_remote_tlbs(kvm);
 }

virt/kvm/kvm_main.c

@@ -621,7 +621,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
 			    struct kvm_userspace_memory_region *mem,
 			    int user_alloc)
 {
-	int r, flush_shadow = 0;
+	int r;
 	gfn_t base_gfn;
 	unsigned long npages;
 	unsigned long i;
@@ -741,8 +741,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
 		if (kvm_create_dirty_bitmap(&new) < 0)
 			goto out_free;
 		/* destroy any largepage mappings for dirty tracking */
-		if (old.npages)
-			flush_shadow = 1;
 	}
 #else  /* not defined CONFIG_S390 */
 	new.user_alloc = user_alloc;
@@ -813,9 +811,6 @@ int __kvm_set_memory_region(struct kvm *kvm,
 	kvm_free_physmem_slot(&old, &new);
 	kfree(old_memslots);
 
-	if (flush_shadow)
-		kvm_arch_flush_shadow(kvm);
-
 	return 0;
 
 out_free: